Whatever happened to holographic storage?

Holographic storage may yet knock magnetic media off its perch

Invented in 1956 by IBM, hard drives have changed little over the last 50 years.

The most obvious changes have been physical size and the amount of data that can be stored: the IBM 350 RAMAC from 1956, was about the size of two refrigerators and could only store 3.75mb of data. A modern desktop drive can store 3Tb in a unit smaller than a paperback book.

While the technology to cram so much data into a tiny space has advanced considerably, the basic principle of a hard drive has remained the same, a rotating disc covered in magnetisable material, spinning past a read/write head.

In fact, the way that data is read from and written to a hard disc's platters is very similar to how data is stored on magnetic tape, a technology that harks all the way back to the 1890s.

Pick pretty much any science fiction book or film involving computers and you'll probably see a reference to holographic storage, often referred to as data cubes. The idea of being able to store tens or thousands of terabytes of information in a device the size of a sugar cube, has long been a dream of many in computing and storage industries, but with the exception of memory cards, high-capacity, portable storage seems to be somewhat lacking.

So what exactly is holographic storage and why is it so desirable? Holographic storage is an optical storage method, in that it uses light to read and write the data. However, unlike existing optical storage technologies, such as CD and DVD, the data is stored in three-dimensions, rather than two.

While DVDs may utilise multiple layers, the laser that reads the information can only do so from one angle. In a holographic storage system, information can, in theory, be stored at multiple depths greatly increasing the storage density.

Need for speed

Another advantage that holographic storage has over conventional hard disk or optical disc storage is speed. Because the discs in a drive are spinning, data is read in a linear fashion. In holographic systems, the data is read in parallel, resulting in much higher data rates.

To store the data, a laser beam is split into two beams, a signal beam and a reference beam. The signal beam passes through a liquid crystal display, which shows a page of binary information, as clear and black boxes.

The beam then travels into a light sensitive polymer or crystal substrate, carrying the information from the LCD. A second beam, called the reference beam, is guided onto a separate path into the light-sensitive substrate, and where the two beams meet, an interference pattern is created, which is stored as a hologram.

By changing the angle of two laser beams, data can be stored within a crystal substrate in multiple levels

To read the data back, the two beams are shone into the substrate at exactly the same angle as was used to create the data, and the hologram is then read by a Charged Couple Device (CCD), similar to that found in a digital camera. By varying the angle of the beams, data can be stored, potentially, in thousands of different layers, depending on the quality of the substrate used.

To read the date back, the reference beam has to be set to the exact same angle, from which the data was recorded

One of the main developers of Holographic storage was InPhase technologies, which produced the first commercial holographic drive and media. However, far from the data cube idea, InPhase's Tapestry system used an optical disc system, which looks similar to a DVD, held inside a caddy (much like DVD-RAM, in fact).